1. Field of the Invention
This invention relates generally to high power factor, pulse width modulated, power controllers, and more specifically to electronic ballasts for discharge lamps.
2. Discussion of the Prior Art
Electronic ballasts function primarily as power supplies for discharge lamps, which represent one of the most difficult loads to control. Problems associated with power factor, efficiency, harmonics, RFI/EMI, system control, soft-starting, fault protection management, reliability, and lamp arc current crest factor, must all be addressed by a modern ballast system.
In an electric power distribution line, the power factor is the ratio of real power (watts) to apparent power (volt-amperes). The optimum value for this ratio is unity (1.00), a value that is obtained only when the line current is sinusoidal and in phase with the line voltage; assuming, of course, that the line voltage is itself sinusoidal. This means that any current component in quadrature with the fundamental, and any components at frequencies other than the fundamental (harmonics), cannot carry power to the load. However, these components contribute to total line losses, and because they add to the current actually required by the user, they mandate the use of heavier wiring and circuit breakers, all of which equates to increased installation costs.
In the past, the main cause of low power factor was phase lag was caused by the inductive characteristic of the electric ballasts. This phase lag accounted for a large portion of the overall load serviced by the electric power companies. In this case, the power factor is equal to the cosine of the phase angle; a power factor of unity results when the angle is zero. Phase lag can be corrected by simply adding the right amount of capacitance in shunt with the offending equipment, as has been done for many years.
More recently, there has been an enormous increase in the number of electronic ballasts incorporating line rectifiers followed by capacitor input filters. As a result, the nature of the problem has changed from strictly a voltage-current-phase relationship to a concern for the effects of harmonics.
The current drawn by these circuits is distinctly non-sinusoidal because the distorted current waveform is the sum of many components of different frequencies, the one at the fundamental line frequency being the useful. The resulting power factor may be as low as 50% under these conditions. Where there are dozens, and more likely hundreds, of electronic ballasts within a single facility, it is imperative for electronic ballast manufacturers to search for some means for improving the low power factor. In addition, there are sensitive medical, laboratory or military environments where the allowable percentage of harmonics and RFI/EMI in the line current are extremely low. In these systems, the need for a high power factor, low harmonic and low RFI/EMI topology is a necessity.
Electronic ballasts have evolved through three distinct generations. First generation electronic ballasts included convention invertor design that provided for the conversion from low frequency V.sub.ac to pulsating V.sub.dc. This conversion was accomplished by means of a rectifier bridge, "bulk" filter capacitance to smooth the pulsating V.sub.dc and then conversion from V.sub.dc to high frequency V.sub.ac by means of a self-oscillating invertor which was coupled to the lamp load. In this generation of electronic ballasts the load draws current as needed from the "bulk" capacitor, while the full-wave rectifier replenishes the capacitor at each half cycle with bursts of current that occur briefly at the voltage peak. In such a system, the output voltage is unregulated, and the input AC current is badly distorted. The weakness of this approach is low power factor, high harmonic distortion of the line current, high RFI/EMI, instant starting of lamps, high lamp arc current crest factor, poor light output regulation, poor system control, high component stress, virtually no fault protection management and poor system reliability.
A second generation electronic ballast included the same basic invertor design except with the addition of large and expensive RFI/EMI filters. The second generation electronic ballasts have all of the weaknesses mentioned for the first generation with the exception of improved power factor and lower RFI/EMI. In addition to all of the other weaknesses mentioned above, high harmonics continue to be a problem with the second generation electronic ballasts.
Third generation electronic ballasts introduced pre-regulator converter topologies that convert low frequency V.sub.ac to a relatively flat V.sub.dc with less distortion to the input line current waveform. The V.sub.dc is typically applied to a "semi-synchronized" or "un-synchronized" high frequency half bridge invertor which is coupled to the lamp load.
Semi-synchronized means that the converter and the invertor are operating at the same frequecy and therefore the converter and invertor are not synchronized due to the bridge topology of the secondary output stage. In order to achieve operation at the same frequency the invertor must switch-on twice for every time the boost converter switches on once. The weakness in this type of semi-synchronized operation is that there is more noise put back on the line. Furthermore, the switching noise generated by the invertor can contaminate the system control circuitry and cause premature failure. Other problems which these electronic ballasts include the lack of both voltage and frequency control during starting and running modes, less efficient peak current mode control of the pre-regulator converter, discontinuous operation of the inductor current and subsequently higher component stress, higher RFI/EMI, higher harmonic distortion of the line current, no invertor dead-time control due to single output drive to the bridge, less than optimum lamp arc current crest factor, and limited applications.
Un-synchronized means that the pre-regulator converter and the output invertor are running at completely different frequencies. In addition to having the weaknesses of the semi-synchronous ballasts mentioned above, other weaknesses include instant starting of lamps, no dimming capabilities, limited fault protection management, and poor reliability. Noise immunity measures within the circuit are more critical because the completely random switching transients generated by the self oscillating invertor have a higher probability of introducing spurious control signals to the system. This typically results in loss of system control and possible component failure. Both un-synchronized and semi-synchronized electronic ballasts change only the frequency to start and control the load. Due to the frequency modulation found in both types of ballast, RFI/EMI filter design optimization is more difficult and costly.
With regard specifically to discharge lamps, there has always been a need to start the lamp as gently as possible, to provide stable operation of the lamp with a lamp arc current crest factor as close to 1.00 as possible, and at the same time to minimize RFI generated by the lamp.